Incandescent lamp with filament consisting of a hexaboride of a rare earth metal



Aug. 27, 1968 Filed July 15, 1965 E. KAU ER 3,399,321

INCANDESCENT LAMP WITH FILAMENT CONSISTING OF A HEXABORIDE OF A RAREEARTH METAL 2 Sheets-Sheet 1 FIG.1

INVENTOR.

ERHARD KAUER BY M Aug. 27, 1968 Filed July 15, 1965' E. KAUER 3,399,321

INCANDESCENT LAMP WITH FILAMENT CONSISTING OF A HEXABORIDE OF A RAREEARTH METAL 2 Sheets-Sheet INVENTOR.

ERHARD KAUER g e e United States Patent av Claims. (01. 313-218 ABSTRACTOF THE DISCLOSURE,

An incandescent lamp employing a filament emitting principally visibleradiation at the operating temperature FILAMENT CON- and consisting of amaterial having a concentration of I a charge carriers between 2X10 and2X10, the charge carriers having -a--mobility at the operatingtemperature of at least 2 cmF/volt-see.

The invention relates to an electric incandescent lamp which employs afilament of a material having a specific selective light emission in thevisible part. of the spectrum.

It has been proposed repeatedly to improve the luminous efficiency ofthermal radiators by using filaments with selective radiationproperties. Such a filament must have emission capacity in the visiblerange approaching unity but for all the other wavelengths, in particularin the near infrared region, it should be very small.

As is known, for example, in incandescent lamps em- 5 ploying a filamentof tungsten, the adverse energy losses, which may be approximately 97%,mainly consist of infrared radiation. This is due to the fact thattungsten has an emission capacity which, in the infrared region, is onlyslightly lower than in the visible region.

As selective radiators only non-metallic, non-conductive orsemi-conductive substances, for example, oxidic materials having highmelting points, in particular ceramic materials which may be colored inaddition are actually known so far. Such radiators require special lampconstructions; in general, a ballast impedance must be used. Forstarting such lamps in operation often heat has to be applied to thefilament externally, for example, in the form of a gas-discharge. As aresult of these and other difficulties lamps with such radiators havenot found universal acceptance. On the other hand, attempts have beenmade to increase the efliciency of tungsten incandescent lamps by higheroperating temperatures, for example, by using transport reactions as isthe case in the tungsteniodine-cycle.

The object of the invention is to use compounds with metallicconductivity which, in contrast with the known metals with high meltingpoints, have a specific selectivity as to their light emission at hightemperatures.

In the investigation which has resulted in the invention it has beenfound that such compounds must have certain properties with respect tothe concentration of the free charge-carriers and the mobility of suchcarriers.

As a result of the absorption of the free charge carriers, the saidcompounds, already in layers having a thickness of approximately l,uwill not be transparent throughout the entire visible region and theinfrared region of the spectrum. In this condition the emission capacityis equal to I-R. In the ideal case it is endeavoured to obtain areflection of unity in the infrared region to make the emission equal to0, while on the other hand in the visible region the reflection shouldbe small and as a result the emission capacity would have to be large.

The reflection R can be expressed by certain parameters of the material.The theory of the optical constants of free 3,399,321 Patented Aug z'i',1968 carriers gives the known relations for the real and the imaginarypart of the coefficient ofrefiection n-f =n, iK:

K is related to the absorption coefiicient at of the by the relationot=41rK In the above relation:

nCoefiicient of refraction of themedium e Dielectric constant of themedium in the ab'sence of free carriers N-Concentration of chargecarriers eElemen tary charge l v v r'n*Effec tive"mass of the chargecarfiers, which is de fined by the relation F =m*dv/dt, where F is theexternal force, for example, of an electric field, and v the velocity ofthe carriers e --Dielectric constant of the vacuum wAngular frequency ofthe light y-Damping constant which is equal to e/,um*, where ,u

is the mobility of the carriers.

From these equations n and K, and therefrom the refiection according tocan be calculated for all wavelengths. A reflection curve is thenobtained as is shown in FIG. 1. Starting at short waves, the reflectiontraverses at least one minimum and then rapidly rises to high values. Itis required that the transition from low to maximum reflection of aselective radiator be effected within a small Wavelength interval. In athermal selective radiator the said transition should lie on theboundary between the visible region and the infrared region, i.e., atapproximately 0.7 ,ull'l which corresponds to an angular frequency of2.7)(10 SEC-1. This frequency is fixed by the condition n K =0 and isobtained.

When 7 is negligibly small, the concentration N of carriers required forthe desired angular frequency can be evaluated from this formula tobe atleast 2.2x 10 cmi- In this evaluation it is assumed that 5 isapproximately 10 and that the effective mass m* is equal to the mass ofhe free electron. Since the effective masses of charge carriers aregenerally smaller than those of the free electron, the saidconcentration of carriers must be considered to be the upper limit.

In experiments it has further been found that the concentration of thefree charge carriers must be at least 2X10 cm. and the mobility ofthecarriers at the operating temperature at least 2 cm. /volt-sec.

It has now been found that the hexaborides of the rare earths andalkaline earth metals, particularly, readily meet the said requirements.This appears from the following table and the figures.

Hexaboride Concentration Mobility cm Reflection side Compound ot thecharge volt-sec. in microns carriers per cm. (pm) 1. 5X l0 l5 0. 65

The invention will be described with reference to the accompanyingdrawing in'which FIGS. 1, 2,3 and 4 are curves showing the reflection ofvarious materials as a function of wave-lengths and FIG. shows a lampaccording to the invention.

FIGURES 1, 2 and 3 show the reflection of NdB PrB and CeB as a functionof the wavelength. In these cases the variation of the reflection showsgreat resemblance. In the direction from short to longer waves at leastone minimum at 0.6 ,um i traversed, after which a steep ascent of thereflection follows.

FIG. 4 shows the reflection curve of LaB and YbB Whereas the LaB behavesas the other hexaborides, the reflection' side of YbB lies atapproximately 1.5 m. The cause hereof must be considered to be theconsiderably lower charge concentration of the YbB (see the table).

Alternatively mixtures of the said compounds may be used. The reflectionside may then be shifted continuously, for example, between the curvesof LaB and YbB shown in FIG. 4.

The filaments can simply be manufactured ceramically, while it is alsopossible to borinate a coiled wire of, for example, lanthanum in knownmanner.

Thus, FIG. 5 shows an incandescent lamp employing a filament of LaB inan envelope 2 containing iodine vapor.

The vaporization or dissociation of the hexa-borides during burning canbe prevented to a great extent by using a cycle, for example, by using avolatile boro-halogen compound, for example, BF BCl so at least the mostvolatile component of the compound is transported back to the filamentduring burning by means of a chemical transport reaction. The filamentswhich according to the invention comprise a selectively radiating bodyneed no ballast impedance or additional means for heating the filament.

What is claimed is:

1. An incandescent lamp employing a selectively radiating filamentemitting principally visible'radiation at a given operating temperatureand consisting of a material having a concentration of charge carriersbetween 2X 10 and 2X10 per cm. and the mobility of the charge carriersat the operating temperature of the radiator being at least 2 cm./volt-sec., said filamentconsisting of a hexaboride of an elementselected from the group consisting of lanthanum and the rare earthmetals. I

2. An incandescent lamp as claimed in claim 1 including means to returnto the filament at least the most volatile component of the compound ofwhich the filament exists by means of a chemical transport reaction.

which the means for returning the volatile component to the filament isa borohalogen compound.

References Cited UNITED STATES PATENTS 3,132,278 5/1964 Collins et al313-222 X 3,134,924 5/1964 Henderson et a1. 313-218 X 3,334,261 8/1967Butler et al 313---227 X JAMES W. LAWRENCE, Primary Examiner.

P. C. DEMEO, Assistant Examiner.

